The invention relates to the field of optical communication, and in particular to high bit-rate optical communication over multimode fibers.
Multimode fibers are commonly used in the optical communication industry. Most multimode fibers used in the communication systems have core diameters of 50 μm or 62.5 μm. Some plastic fibers may have larger core diameters. These fibers may have step-index or radially varying graded-index profiles. Typical bandwidth-distance product (BDP) for these fibers in which all modes are excited is in the range of 200–2000 MHz.km with the newer graded index fibers occupying the higher end of the BDP range. These fibers were originally deployed for ≦1.25 Gbps communication channels.
Many approaches are being pursued to achieve higher BDP with an old or a new MM fiber. The most promising approach involves launching laser light such that only a subset of the modes of the fiber are excited. Carefully chosen subsets of modes have smaller modal dispersion than the conventional BDP of the fiber which is often calculated assuming that the laser had excited all the modes. This effectively increases BDP and is shown in the laboratory to increase the effective bandwidth (EB) of the MM fiber so that one can meet the data transport needs.
One of the most popular of these Restricted Mode Launch (RML) schemes is to launch from a SM fiber with a 6–10 μm mode diameter into the MM fiber with an offset of 10–15 μm from the center. This has been seen as the most robust means of exciting the subset of modes for a wide variety of fiber types. The received signal is collected in its entirety on a detector. Even with the RML, there is sufficient modal dispersion of the excited modes so as to limit the data communication rates to below the needs of the user. Thus, as the data rate is increased, one can see “eye closure” or that the impulse response of the fiber channel is spread over multiple unit intervals of data rate. For example, a 62.5 micron core fiber with index perturbed from an ideal square law index profile may not support 10 Gbps over a distance of 500 m. This makes the direct recovery of the signal difficult or impossible with low BER. Thus, the received signal is then electronically processed to recover the original signal with low BER. The methods for “cleaning up” may use combination of error correcting codes (ECC) and signal processing to provide electronic modal dispersion compensation.
Most of the reported effort to increase the EB has been spent on exciting a subset of modes. Much work in this area has resulted in identification of a group of modes that have acceptable BDP for high speed transport. Ideally, one needs to pick a robust group with characteristics such as (i) the modes from this group propagate along the fiber with little excitation of the modes from the other groups, (ii) completely mixed within the group so that the details of the excitation is averaged out producing reliable output on the receiver, and (iii) works with most installed fiber types. It has proven difficult to satisfy all of the above so that a simple and universal electronic dispersion compensator can be designed. At present, offset mode launch together with an EDC is considered a possible solution.
There is a need for using these legacy fibers to transport higher bandwidth signals in the range of 4–10 Gbps for a distance of at least 300 m. Thus, one needs a BDP of 900–2100 MHz.km. These fibers can be preferred over single mode fibers because of ease of coupling, familiarity, and new installations.